There's really no magic number for maximum number of generations before you have to start over - there's a Michigan brewery that's re-used their yeast over 400 times now. It's more important with how you maintain your cultures and the kinds of beers from which you harvest yeast. If you notice your yeast behaving differently, and you're not happy with the result, then it's time to start fresh.

The White/Jamil Z. book mentions 8 generations with ale yeast, and 4 generations with lager yeast. Of course with good technique YMMV. Unfortunately I don't brew often enough to hit their recommended "use within 2 weeks" sweet spot.

Its good to know, and post... with summer and the heat, I can see the LHBS run out of their yeast, so the home brewer should know, with good sanitation practices, you can brew quite a few batches and not worry too much... Sneezles61PS... almost lager season too!

I repitched 3724 slurry with a starter on Sunday for a cherry saison and it took off within a few hours. With this strain, subsequent pitches are reportedly more vigorous and less likely to suffer the infamous 3724 stall.....we'll see.

Over time this approach is not only easier but probably cost effective. The SNSYS process is pretty easy(shaken not stirred yeast starter) and I tend to use this because I don' t brew often enough, as I said earlier, to do a straight repitch. Some of my saved yeast is getting a little long in the tooth. I keep asking my wife to avoid stuffing things into the 4th shelf down in the fridge!